The present invention relates to a lowpass filter; and, more particularly, to a lowpass filter for effectively suppressing a spurious in a stop-band, which is generated from a filter operating in a high frequency band including a Radio Frequency (RF) band or a microwave band.
Generally, a conventional lowpass filter is composed of a transmission line in the Radio Frequency RF or the microwave band and has a spurious characteristic due to a periodic characteristic of the transmission line in a stop-band.
Referring to FIGS. 1 and 2, in a conventional lowpass filter implemented by using microstrips, an input port 210 and an output port 270 are microstrips with a characteristic impedance as an impedance of an input/output ports.
Capacitors 120, 140 and 160 in FIG. 1 are implemented by using Open-ended microstrips 220, 240 and 260 in FIG. 2. Capacitances corresponding to the open-ended microstrips 220, 240 and 260 are determined by a line-width, a line-length and an operation frequency of the microstrips.
Microstrip lines 230 and 250 connect the open-ended microstrips 220 to the open-ended microstrips 240 and the open-ended microstrips 240 to the open-ended microstrips 260, respectively. The microstrip lines 230 and 250 at FIG. 2 are operated as inductors 130 and 150 of FIG. 1 at especially cutoff frequency.
A circuitry employing with microstrips in FIG. 2 has the similar result with a circuit in FIG. 1 in a low frequency band. However, in a high frequency band, the results of the circuitry in FIG. 2 show a difference results with a circuit in FIG. 1. Such a difference in results has been a problem of the conventional lowpass filter shown in FIG. 2.
Referring to FIG. 3, xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d denote an insertion loss and a reflection loss of the conventional lowpass filter shown in FIG. 2.
A dotted line in a graph of FIG. 3 represents a spurious response of the conventional lowpass filter. As shown, un-wanted spurious characteristics appear in the stop-band. The spurious characteristics in the stop-band are due to a periodical impedance change characteristic of microstrip structure of the conventional lowpass filter.
For suppressing such a problem, conventional techniques have been introduced and described in an article by T. Garvens, xe2x80x9cMicrowave LPF design for RF Applicationxe2x80x9d, 1995, RF Expo, pp. 147-159, (1995); by J. W. Sheen, xe2x80x9cA compact semi-lumped lowpass filter for harmonics and spurious suppressionxe2x80x9d, IEEE Microwave and Guided Wave Letters, vol, 10, pp.392-93, (March 2000); by Y. Qian and et al., xe2x80x9cMicrowave applications of photonic band-gap(PBG) structuresxe2x80x9d, 1999 IEEE International Microwave Symposium, (June, 1999).
Garvens introduces a method for improving a spurious response by avoiding a periodical structure according to a specified frequency band. By changing a shape of microstrips in a microstrips type lowpass filter, the periodical characteristic with respect to frequency can be avoided. However, the method only improves a spurious response characteristic in a specified frequency band but not in a broad frequency band. The method cannot be implemented at the broad frequency band.
Sheen proposes a semi-lumped type lowpass filter for improving a spurious response by altering a harmonic frequency according to periodical characteristic of microstrip with respect to a frequency. For avoiding the spurious response, the semi-lumped type lowpass filter is proposed, which is composed of lumped elements and microstrips in parallel connection. The semi-lumped lowpass filter improves the spurious response characteristic of the conventional lowpass filter in a narrow frequency band; however, in broad frequency band, the semi-lumped type lowpass filter cannot improves spurious response characteristic. Qian et al. introduces a method for improving a spurious response by setting the stop-band as a specified frequency band by applying a constant defect to a groundside of microstrips. The method improves the spurious response at the specific frequency band. However, beyond the aimed the specific frequency band, its effectiveness is disappeared or another spurious characteristic may appear in another frequency band.
It is, therefore, an object of the present invention to provide a lowpass filter for effectively suppressing a spurious characteristic due to a periodic characteristic of a transmission line with respect to frequency using an open-ended microstrips employing thick/thin resistors.
In accordance with an aspect of the present invention, there is provided a lowpass filter, including: an input port microstrip element; an output port microstrip element; a first open-ended microstrip element connected to the input port microstrip element, having resistance elements both located in predetermined portion of the first open-ended microstrip line and being operated as a capacitor; a second open-ended microstrip element connected to the output port microstrip element, having resistance elements both located in predetermined portion of the microstrip line and being operated as a capacitor; a third open-ended microstrip element connected between the first open-ended microstrip element and the second open-ended microstrip element, having resistance both located in predetermined portion of the microstrip line, and being operated as a capacitor; a first connector microstrip element for connecting the first open-ended microstrip element and the second open-ended microstrip element and being operated as an inductor; and a second connector microstrip element for connecting the second open-ended microstrip element and the third open-ended microstrip element and being operated as an inductor.
The lowpass filter including microstrip have at least one open-ended microstrip element of which a predetermined portion is removed and distributed resistances inserted both located in predetermined portions of the open-ended microstrip.
The open-ended micro strip element of the lowpass filter includes distributed elements and distributed resistances connected to each other in series and operated as a capacitor, wherein distributed resistance increases impedance of the microstrips in case of a low frequency and decreases the impedance of the microstrips in case of a high frequency.